Epidemiological and case-control studies have linked a high fat diet to increased incidence and mortality from breast cancer. Using a randomized controlled prospective study design, the addition of a low fat diet (20% of dietary calories) on disease relapse and overall survival in stage II breast cancer patients receiving chemotherapy after primary surgical treatment will be determined. Over a three year period 150 breast cancer patients will be accrued, and over the final two years, follow-up will assess long term dietary compliance and impact of dietary intervention on disease, recurrence and survival. Adjuvant chemotherapy will be given at six referring hospitals with all patients receiving dietary intervention at a central location (UCLA). The first 50 patients will be part of an initial 10-month feasibility trial, with subsequent study modifications applied to the remaining patients. After determination of eligibility, patients will be randomized to either the control group (no alteration in dietary fat content) or study group (institution of a 20% dietary fat content). A chemotherapy regimen utilizing i.v. cytoxan, methotrexate, and 5-fluorouracil (CMF) will be given every three weeks for six months. The frequency of dietician and physician visits will be the same for all patients. Both control and study patients will receive counseling directed at achieving the RDA for major macro- and micro nutrients. All patients will have a vitamin/mineral preparation to this end. Nutritional assessment, quality control measures, and patient feedback will involve 24-hour recalls, food frequency questionnaires, body weights, skin-fold analyses, a daily fat and cholesterol score, and lipid analysis. To provide the optimum method for instituting a dietary change that will persist (i.e., lifestyle change), the study group (20% fat content diet) will receive behavior modification sessions and an exercise program as part of a multidisciplinary effort in the UCLA Center for Health Enhancement. The control group (unchanged fat content diet) will be seen separately in the UCLA Cancer Center, where they will be instructed in a well-balanced diet based on the four basic food groups. The acute and chronic influence of chemotherapy effects on dietary composition will be determined. Practical quality control measures to facilitate dietary compliance assessment will be developed. Following completion of this trial, the role of low fat dietary intervention in the adjuvant treatment of breast cancer will be defined.